246 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



After this maximum is reached the arm remains more or less of 

 the same volume for a certain period or else diminishes in volume 



very gradually. Shortly before waking, 

 however, the arm begins to diminish 

 more rapidly in size, owing doubt- 

 less to the contraction of its blood- 

 vessels; so that at the time of awak- 

 ing it has practically the same volume 

 as at the beginning of sleep. If, on 

 the basis of Mosso's experiments, 

 quoted above, we assume that the 

 blood-flow in the brain stands in a 

 reciprocal relation to that in the arm, 

 this curve may be taken to indicate 

 that before and after the onset of sleep 

 the blood-flow through the brain di- 

 minishes rapidly to a certain point 

 and that before awaking the blood-flow 

 begins to increase again until it reaches 

 normal proportions. 



Effect of Sensory Stimulation. 

 That sensory stimuli of various kinds 

 affect a sleeping individual without 

 entirely awaking him is shown by the 

 movements that may be caused in this 

 way, and also by the nature of the 

 dreams which may be provoked. It is 

 very interesting to find from plethys- 

 mographic observations that all kinds 

 of sensory stimulations from with- 

 out and from within are liable to 

 affect the circulation of the blood 

 during sleep. As shown by the plethys- 

 mograph, the volume of the arm dimin- 

 ishes more or less in proportion to the 

 * intensity of the stimulus, and the 

 probable interpretation of this fact is 

 that the sensory stimulus acts reflexly 

 upon the vasomotor center in the 

 medulla and causes through it a con- 

 traction of the blood-vessels. In the 

 curve shown in Fig. 109 most of the 



irregularities were traceable to causes of this kind, noises 

 in the building or street or other sensory stimuli. The 

 same fact is exhibited in a striking way by the curves given in 



